Guarantor Condensed Consolidating Financial Statements (Condensed Consolidating Statements Of Cash Flows) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Net cash provided by/(used in) operating activities	$ 247.2	$ 146.0
Investing activities:
Purchase of investments by consolidated investment products	(1,584.6)	(2,075.3)
Proceeds from sale of investments by consolidated investment products	1,537.0	2,300.9
Returns of capital in investments of consolidated investment products	(44.9)	75.5
Purchase of other investments	(63.4)	(61.4)
Net cash (used in)/provided by investing activities	(186.9)	229.9
Financing activities:
Purchases of treasury shares	(150.0)	(333.0)
Dividends paid	(133.7)	(108.5)
Capital invested into consolidated investment products	19.4	32.5
Capital distributed by consolidated investment products	(35.5)	(134.9)
Repayments of debt of consolidated investment products	145.7	(246.3)
Net borrowings/(repayments) under credit facility	272.0	268.0
Repayments of senior notes	(215.1)
Net cash provided by/(used in) financing activities	(73.1)	(509.5)
(Decrease)/increase in cash and cash equivalents	(12.8)	(133.6)
Foreign exchange movement on cash and cash equivalents	3.8	14.6
Cash and cash equivalents, beginning of period	727.4	740.5
Cash and cash equivalents, end of period	718.4	621.5
Guarantors [Member]
Net cash provided by/(used in) operating activities	400.0	(243.0)
Investing activities:
Purchase of investments by consolidated investment products	0	0
Proceeds from sale of investments by consolidated investment products	0	0
Returns of capital in investments of consolidated investment products	0	0
Purchase of other investments	(4.3)	(1.2)
Other net increases/(decreases) in investing activities	(17.1)	(20.9)
Net cash (used in)/provided by investing activities	(21.4)	(22.1)
Financing activities:
Purchases of treasury shares	0	0
Dividends paid	(436.6)	0
Capital invested into consolidated investment products	0	0
Capital distributed by consolidated investment products	0	0
Repayments of debt of consolidated investment products	0	0
Net borrowings/(repayments) under credit facility	50.0	268.0
Repayments of senior notes	0
Other net increases/(decreases) in financing activities	0	0
Net cash provided by/(used in) financing activities	(386.6)	268.0
(Decrease)/increase in cash and cash equivalents	(8.0)	2.9
Foreign exchange movement on cash and cash equivalents	0	0
Cash and cash equivalents, beginning of period	20.6	12.4
Cash and cash equivalents, end of period	12.6	15.3
Non-Guarantors [Member]
Net cash provided by/(used in) operating activities	227.6	77.5
Investing activities:
Purchase of investments by consolidated investment products	(1,584.6)	(2,075.3)
Proceeds from sale of investments by consolidated investment products	1,537.0	2,300.9
Returns of capital in investments of consolidated investment products	(44.9)	75.5
Purchase of other investments	(3.3)	81.3
Other net increases/(decreases) in investing activities	(35.3)	(57.1)
Net cash (used in)/provided by investing activities	(131.1)	325.3
Financing activities:
Purchases of treasury shares	0	0
Dividends paid	(241.5)	(194.9)
Capital invested into consolidated investment products	19.4	32.5
Capital distributed by consolidated investment products	(35.5)	(134.9)
Repayments of debt of consolidated investment products	145.7	(246.3)
Net borrowings/(repayments) under credit facility	0	0
Repayments of senior notes	0
Other net increases/(decreases) in financing activities	11.9	2.8
Net cash provided by/(used in) financing activities	(100.0)	(540.8)
(Decrease)/increase in cash and cash equivalents	(3.5)	(138.0)
Foreign exchange movement on cash and cash equivalents	3.8	14.6
Cash and cash equivalents, beginning of period	703.0	725.9
Cash and cash equivalents, end of period	703.3	602.5
Issuer [Member]
Net cash provided by/(used in) operating activities	(14.3)	63.8
Investing activities:
Purchase of investments by consolidated investment products	0	0
Proceeds from sale of investments by consolidated investment products	0	0
Returns of capital in investments of consolidated investment products	0	0
Purchase of other investments	(1.6)	(116.0)
Other net increases/(decreases) in investing activities	8.3	51.4
Net cash (used in)/provided by investing activities	6.7	(64.6)
Financing activities:
Purchases of treasury shares	0	0
Dividends paid	0	0
Capital invested into consolidated investment products	0	0
Capital distributed by consolidated investment products	0	0
Repayments of debt of consolidated investment products	0	0
Net borrowings/(repayments) under credit facility	222.0	0
Repayments of senior notes	(215.1)
Other net increases/(decreases) in financing activities	0	0
Net cash provided by/(used in) financing activities	6.9	0
(Decrease)/increase in cash and cash equivalents	(0.7)	(0.8)
Foreign exchange movement on cash and cash equivalents	0	0
Cash and cash equivalents, beginning of period	3.1	1.1
Cash and cash equivalents, end of period	2.4	0.3
Parent Company [Member]
Net cash provided by/(used in) operating activities	312.0	440.0
Investing activities:
Purchase of investments by consolidated investment products	0	0
Proceeds from sale of investments by consolidated investment products	0	0
Returns of capital in investments of consolidated investment products	0	0
Purchase of other investments	(54.2)	(25.5)
Other net increases/(decreases) in investing activities	13.1	19.3
Net cash (used in)/provided by investing activities	(41.1)	(6.2)
Financing activities:
Purchases of treasury shares	(150.0)	(333.0)
Dividends paid	(133.7)	(108.4)
Capital invested into consolidated investment products	0	0
Capital distributed by consolidated investment products	0	0
Repayments of debt of consolidated investment products	0	0
Net borrowings/(repayments) under credit facility	0	0
Repayments of senior notes	0
Other net increases/(decreases) in financing activities	12.2	9.9
Net cash provided by/(used in) financing activities	(271.5)	(431.5)
(Decrease)/increase in cash and cash equivalents	(0.6)	2.3
Foreign exchange movement on cash and cash equivalents	0	0
Cash and cash equivalents, beginning of period	0.7	1.1
Cash and cash equivalents, end of period	0.1	3.4
Adjustments [Member]
Net cash provided by/(used in) operating activities	(678.1)	(192.3)
Investing activities:
Purchase of investments by consolidated investment products	0	0
Proceeds from sale of investments by consolidated investment products	0	0
Returns of capital in investments of consolidated investment products	0	0
Purchase of other investments	0	0
Other net increases/(decreases) in investing activities	0	(2.5)
Net cash (used in)/provided by investing activities	0	(2.5)
Financing activities:
Purchases of treasury shares	0	0
Dividends paid	678.1	194.8
Capital invested into consolidated investment products	0	0
Capital distributed by consolidated investment products	0	0
Repayments of debt of consolidated investment products	0	0
Net borrowings/(repayments) under credit facility	0	0
Repayments of senior notes	0
Other net increases/(decreases) in financing activities	0	0
Net cash provided by/(used in) financing activities	678.1	194.8
(Decrease)/increase in cash and cash equivalents	0	0
Foreign exchange movement on cash and cash equivalents	0	0
Cash and cash equivalents, beginning of period	0	0
Cash and cash equivalents, end of period	0	0
Consolidated [Member]
Net cash provided by/(used in) operating activities	247.2	146.0
Investing activities:
Purchase of investments by consolidated investment products	(1,584.6)	(2,075.3)
Proceeds from sale of investments by consolidated investment products	1,537.0	2,300.9
Returns of capital in investments of consolidated investment products	(44.9)	75.5
Purchase of other investments	(63.4)	(61.4)
Other net increases/(decreases) in investing activities	(31.0)	(9.8)
Net cash (used in)/provided by investing activities	(186.9)	229.9
Financing activities:
Purchases of treasury shares	(150.0)	(333.0)
Dividends paid	(133.7)	(108.5)
Capital invested into consolidated investment products	19.4	32.5
Capital distributed by consolidated investment products	(35.5)	(134.9)
Repayments of debt of consolidated investment products	145.7	(246.3)
Net borrowings/(repayments) under credit facility	272.0	268.0
Repayments of senior notes	(215.1)
Other net increases/(decreases) in financing activities	24.1	12.7
Net cash provided by/(used in) financing activities	(73.1)	(509.5)
(Decrease)/increase in cash and cash equivalents	(12.8)	(133.6)
Foreign exchange movement on cash and cash equivalents	3.8	14.6
Cash and cash equivalents, beginning of period	727.4	740.5
Cash and cash equivalents, end of period	$ 718.4	$ 621.5